This disclosure relates to phase conjugation of distorted wavefronts. In particular, it relates to phase conjugation of distorted wavefronts that result from optical phase mismatch between elements of a fiber laser array. If the wavefronts of beams in a fiber laser array cannot be conjugated, the resulting distortion, sometimes referred to as “piston” error, prevents a uniform wavefront from being achieved at the output of the laser. This reduces power output from the laser, and may cause the laser to shut down.
Recent research has explored different ways to combine multiple fiber lasers or amplifiers to achieve power scaling while maintaining near diffraction limited performance. This research has taken several approaches. These approaches include both coherent and spectral combining.
Passive coherent phasing of laser arrays offers a simpler approach but has met with only limited success. Current passive coherently-combined fiber laser schemes are limited to a coherent brightness gain of approximately 8-12 for large arrays. Some further improvements such as an intensity dependent index (Kerr) nonlinearity have shown modest expected maximum improvement, with coherent brightness gain saturating at 10-14, depending on the strength of the nonlinearity.
This phenomenon has been confirmed theoretically and experimentally. Schemes employing stimulated Brillouin scattering phase conjugate mirrors have been proposed, however, they require a master oscillator as well as a Faraday rotator medium, which cause system engineering difficulties.
Thus, there remains a need for a method and apparatus that provide improved phase conjugation of distorted wavefronts to yield improved coherent brightness gains that facilitate utilization of such output in directed energy and similar applications. In particular, improvements in passive phase conjugation of distorted wavefronts are needed.